oop - how to use a variable of one class in another in php -

I created a class named to connect to the server

  class db {private $ Host = 'localhost'; Personal $ username = 'root'; Private $ password = ''; Private $ DBC; Public Function Connect () {$ this- & gt; Dbc = mysql_connect_db ($ this-> host, $ this- & gt; username, $ this- & gt; password); }   
 And in index.php I have created the object of class 'db'  
  $ con = new db; $ Election & gt; Connect ();   
 And the problem is that I created a query by  
  mysql_query ($ query, "host connection parameter");   
 And I do not know how to pass the second parameter for mysql_query () which is a host connection parameter. I tried typing  
  mysql_query ($ query, $ election> connect () -> dbc);   
 But that did not work. Anyone know how to fix this   
 
 
 Modify your class in this way so that you Can execute questions.  
  class db {private $ host = 'localhost'; Personal $ username = 'root'; Private $ password = ''; Private $ DBC; Public Function Connect () {$ this- & gt; Dbc = mysql_connect_db ($ this-> host, $ this-> username, $ this-> password); } Public function query ($ sql) {return mysql_query ($ sql, $ this-> dbc); }   
 Then you can use  
  $ db = new db (); $ Db- & gt; Connect (); $ RS = $ db- & gt; Query ("SELECT .....");